1. Field of the Invention
The invention relates to an optical film, an illumination device, and an image display device. Specifically, an optical film that can suppress an oblique tint change in a case where the optical film is incorporated into a liquid crystal display device, an illumination device using this optical film, and an image display device.
2. Description of the Related Art
A flat panel display such as a liquid crystal display device (hereinafter, also referred to as LCD) has small power consumption and the use thereof is widened year by year to a space saving image display device.
In a recent flat panel display market, development for power saving by performance improvement in both uses of a use for a large size mainly as a use for a TV and a use for a small size as a use for a tablet PC or a smart phone has progressed.
In order to cause light exit from the light source to be effectively incident on a liquid crystal cell or the like, a technique of improving luminance by collecting outgoing light in a front direction by a light collecting element such as a prism sheet has been suggested.
For example, JP2008-250333A discloses a light collecting element that does not have a pattern structure when being optically observed from a surface side and that is different from a light collecting element due to a refractive index difference between two types of materials including the air and an interface structure, the light collecting element includes at least one polarization reflecting layer (circular polarization reflection layer) and a phase difference layer, the polarization reflecting layer is a circular polarization reflection layer that has selective reflection at a visible light wavelength, transmits certain circularly polarized light, and selectively reflects reverse circularly polarized light, and the phase difference layer transmits light at an angle near a normal direction that does not have a selective reflection wavelength in the range of 380 to 780 nm without change and changes a polarization state of light at an angle tilted from the normal direction by a phase difference, a surface light source and a liquid crystal display device using this collecting element. JP2008-250333A discloses that the outgoing light as polarized light that particularly does not have dependency in an azimuthal angle in a light collecting function can be extracted by the light collecting element having this configuration.
JP2008-250333A discloses that it is preferable that a phase difference layer is arranged between at least two circular polarization reflection layers in which wavelength ranges of selective reflection of the polarized light are overlapped with each other. Particularly, in an example of JP2008-250333A, a light collecting element in which a phase difference layer is arranged between two circular polarization reflection layers having the same wavelength ranges of the selective reflection is obtained by using a layer obtained by fixing a cholesteric liquid crystalline phase as the circular polarization reflection layers.
It is suggested to provide a reflective polarizer between the backlight and the backlight side polarizing plate in accordance with power saving of backlight. The reflective polarizer is an optical element that transmits only light that vibrates in a specific polarization direction among light incident in various directions while vibrating and reflects light that vibrates in the other polarization directions. The reflective polarizer is expected to increase luminance (degree of brightness per unit area of light source) by resolving low light efficiency of LCD as core components of low power LCD in accordance with increase of mobile equipment and low power consumption of home appliances.
For example, JP2003-279739A discloses an optical film in which at least one cholesteric liquid crystal layer (1) and a phase difference layer (2) are laminated, and in which, in the phase difference layer (2), in a case where main refractive indexes in two in-plane directions are nx1 and ny1 (here, nx1≥ny1), a refractive index in a thickness direction is nz1, and a thickness is Dre (nm), an in-plane phase difference: (nx1−ny1)×Dre is 5 nm or less, and in a case where an ordinary light refractive index of the cholesteric liquid crystal layer (1) is no, an extraordinary light refractive index is ne, and a thickness of the cholesteric liquid crystal layer is Dch (nm), when a virtual thickness direction phase difference calculated from these is RZch=Dch×(ne−no)/2, and an average value of in-plane refractive indexes of the phase difference layer (2) is np={(nx1+ny1)/2}, a thickness direction phase difference: RZre=Drex(np−nz) calculated from these has a relationship of RZre/(−RZch)=0.05 to 0.35, and an illumination device and an image display device using these. According to JP2003-279739A, in the configuration, it is possible to provide an optical film that can be used as a luminance enhancement film obtained by applying a phase difference layer to a cholesteric liquid crystal layer, and that has satisfactory visibility in an image display device such as a liquid crystal display device, not only in a case of being seen from a front surface but also even in a case of being obliquely seen.
In an example of JP2003-279739A, a laminate of cholesteric liquid crystal layers (circular polarization reflection layers) of which center wavelengths of the reflection bands are different from each other, a phase difference layer, and an absorptive polarizer are laminated in this order, and a case where a phase difference layer is arranged between two circular polarization reflection layers is not disclosed.